Rotor and method for welding an element of a rotor

ABSTRACT

A method of forming a rotor comprises the step of welding a rotor element. The weld metal comprises: from 0.04 to 0.1% carbon, from 0 to 0.5% silicon, from 0.1 to 0.6% manganese, from 0 to 0.01% sulphur, from 0 to 0.03% phosphorous, from 1.9 to 2.6% chromium, from 0.05 to 0.3% molybdenum, from 0.2 to 0.3% vanadium, from 0.02 to 0.08% niobium, from 1.45 to 2.1% tungsten, from 0 to 0.03% nitrogen, from 0.0005 to 0.006% boron and from 0 to 0.03% aluminium. The rotor element may be formed from steel which comprises from 0.15 to 0.35% carbon, from 0 to 0.3% silicon, from 0.2 to 1% manganese, from 0 to 0.03% sulphur, from 0 to 0.03% phosphorous, from 0.3 to 1% nickel, from 0.7 to 1.50% chromium, from 0.5 to 1.2 % molybdenum, and from 0.2 to 0.4% vanadium.

[0001] The invention relates to a rotor and a method for welding anelement of a rotor. Particularly, but not exclusively, the inventionrelates to a low alloy, high temperature rotor for use in a turbine anda welding method, which method can be used in assembling components ofthe rotor together during manufacture or for repairing the rotorpost-manufacture.

[0002] Turbine rotors are typically constructed from low alloy steel. Inone known construction, the rotor comprises a monobloc body formed froma single forging. In another known construction, the rotor bodycomprises a series of individual disks, which are welded together. Ineach case, blades are disposed upon the periphery of the body by asuitable retaining mechanism, of which there are various types. Theinvention is particularly applicable to these types of rotor, but othertypes, of which there are many, are not excluded.

[0003] Any metal article subjected to stress over a period of time willexperience what is known as “creep”. Creep is the plastic deformation ofthe metal over that period and is therefore time-dependent strain. Creepis extremely sensitive to temperature, and sensitivity increases rapidlywith rising temperature. After a period of time, creep will ultimatelyconclude by the generation of a fracture.

[0004] Whilst the severe consequences of a fracture should not beunderestimated, the situation is mitigated by the fact that it ispossible to predict the progression of creep within a rotor with a gooddegree of accuracy, providing the operating conditions for the rotor areknown and controlled. The consequence is that a “creep-life” for a rotorcan be predicted, therefore enabling the period of time before a failureshould occur to be established. This means that the useful working-lifeof the rotor (the ‘service life’) can be determined in advance, therebyensuring that it is decommissioned or repaired before a failure occurs.

[0005] It is desirable for the creep-life of both the initiallymanufactured rotor and the repaired rotor to be as long as possible.Furthermore, it is highly desirable that the creep-life can be predictedwith a high degree of accuracy, not only to ensure that a fracture orfailure is avoided, but also to ensure that the service-life (which willalways be shorter than the creep-life, the difference being a safetymargin) will be relatively close to the creep-life of a particularrotor, thereby avoiding any unnecessary redundancy of use.

[0006] A rotor that is approaching the end of its service life isnormally repaired by a welding process. Although this invention ishighly applicable to a welding process used in the initial manufactureof a rotor, it has particular applicability to a repair process andfurther discussion will therefore concentrate on a repair process.

[0007] The post-repair rotor will have its creep strength affected notonly by the parent metal (from which the rotor was originallymanufactured), but also the metal used in the production of the repairweld. It is therefore necessary to select a weld metal that will providesufficiently adequate creep characteristics following the repair. It is,however, also necessary to take into account other characteristics ofthe post-repair rotor and this will significantly include the thermalexpansion coefficients of the various integers of the rotor. If theco-efficient of thermal expansion of the weld metal is significantlydifferent from that of the parent metal, distortions and additionaloperating stresses will occur, both of which would not only affect thefurther service-life of the rotor, but also complicate the prediction ofthe creep-life, requiring, at least, a greater error margin to be builtinto the measurements and predictions that would need to be made.

[0008] With the foregoing requirements in mind, a number of weld metaltypes have, for example, been used for a typical rotor type,manufactured from 1% CrMoV low alloy steel.

[0009] A first such weld metal type has a creep strength which is atleast as great as that of the parent metal, but is a higher alloymaterial, with a lower co-efficient of thermal expansion; one example ofthis is 12% CrMoV used in association with a 5% Cr weld metal layerbetween the parent metal and the 12% CrMoV weld metal. A further knownalloy material has physical properties which are similar to that of theoriginal parent rotor material, but with lower creep strength, forexample low carbon 1% CrMoV or 2% CrMo.

[0010] The first of these types, whilst having a sufficiently adequatecreep strength, introduces uncertainty by virtue of the thermal effectsof having a lower co-efficient of thermal expansion relative to therotor steel. The latter suffers from having a lower creep strength thanthe rotor steel, so that the prolongation of the rotor life would belimited.

[0011] It is therefore an object of the invention to provide a processwhich results in a rotor that does not have the creep life normallyattributable to the parent metal substantially shortened as aconsequence of the creep-life of the weld metal and in which theco-efficient of thermal expansion of the parent metal and weld metal arecomparably similar or identical.

[0012] According to a first aspect of the invention, there is provided amethod of forming a rotor comprising the steps of: (i) providing a rotorelement formed from steel; and (ii) welding the rotor element, using awelding process employing a weld metal which comprises: from 0.04 to0.1% carbon, from 0 to 0.5% silicon, from 0.1 to 0.6% manganese, from 0to 0.01% sulphur, from 0 to 0.03% phosphorous, from 1.9 to 2.6%chromium, from 0.05 to 0.3% molybdenum, from 0.2 to 0.3% vanadium, from0.02 to 0.08% niobium, from 1.45 to 2.1% tungsten, from 0 to 0.03%nitrogen, from 0.0005 to 0.006% boron and from 0 to 0.03% aluminium.

[0013] This method can be used during manufacture or repair and canresult in a rotor that has a good creep life that is predictable with ahigh degree of certainty, thereby enabling the service life of the rotorto be extended to a point relatively close to the actual end of thecreep life of the rotor. The method also provides for a rotor havingconsistent thermal expansion coefficients, thereby avoiding undesirableoperating stresses on the rotor.

[0014] Preferably, the said weld metal comprises at least 0.06% carbon.It is also preferred that the said weld metal comprises at least 0.3%manganese. It is further preferred that the said weld metal comprises0.005% or less sulphur. It is still further preferred that the said weldmetal comprises at least 1.7% tungsten. It is further preferred that thesaid weld metal comprises at least 0.04% niobium. It is furtherpreferred that the said weld metal comprises 0.02% or less nitrogen. Itis yet further preferred that the said weld metal further comprises 0.5%or less nickel.

[0015] In one particularly preferred embodiment, the said weld metalcomprises substantially 0.075% carbon, 0.2% silicon, 0.5% manganese,0.001% sulphur, 0.017% phosphorous, 2.2% chromium, 0.1% molybdenum, 0.1%nickel, 0.23% vanadium, 0.06% niobium, 0.05% titanium, 1.9% tungsten,0.009% nitrogen, 0.003% boron and 0.02% aluminium.

[0016] The rotor element may be formed from steel which comprises from0.15 to 0.35% carbon, from 0 to 0.3% silicon, from 0.2 to 1% manganese,from 0 to 0.03% sulphur, from 0 to 0.03% phosphorous, from 0.3 to 1%nickel, from 0.7 to 1.50% chromium, from 0.5 to 1.2% molybdenum, andfrom 0.2 to 0.4% vanadium.

[0017] In one particularly preferred embodiment, the rotor element isformed from steel comprising substantially 0.25% carbon, 0.23% silicon,0.64% manganese, 0.005% sulphur, 0.01% phosphorous, 0.56% nickel, 0.8%chromium, 0.78% molybdenum, and 0.35% vanadium.

[0018] The method may comprise providing a second rotor element having acomposition substantially the same as the said rotor element and weldingthe said second rotor element to the said rotor element using the saidweld metal.

[0019] The said welding process may be a submerged-arc welding process.

[0020] The said method may further comprise a step of machining a rotorcomponent to form at least one of the said rotor elements.

[0021] The said method may also comprise a step of machining the saidweld metal after the weld has been formed.

[0022] According to a second aspect of the invention there is provided arotor for a steam turbine, comprising a rotor element and weld metalwelded to the said rotor element; wherein the weld metal comprises: from0.04 to 0.1% carbon, from 0 to 0.5% silicon, from 0.1 to 0.6% manganese,from 0 to 0.01% sulphur, from 0 to 0.03% phosphorous, from 1.9 to 2.6%chromium, from 0.05 to 0.3% molybdenum, from 0.2 to 0.3% vanadium, from0.02 to 0.08% niobium, from 1.45 to 2.1% tungsten, from 0 to 0.03%nitrogen, from 0.0005 to 0.006% boron and from 0 to 0.03% aluminium.

[0023] Preferably, the said weld metal comprises at least 0.06% carbon.It is further preferred that the said weld metal comprises at least 0.3%manganese. It is still further preferred that the said weld metalcomprises 0.005% or less sulphur. It is yet further preferred that thesaid weld metal comprises at least 1.7% tungsten. In one preferredembodiment, the said weld metal comprises at least 0.04% niobium. It isfurther preferred that the said weld metal comprises 0.02% or lessnitrogen. It is yet further preferred that the said weld metal furthercomprises 0.5% or less nickel.

[0024] In one particularly preferred embodiment, the said weld metalcomprises substantially 0.075% carbon, 0.2% silicon, 0.5% manganese,0.001% sulphur, 0.017% phosphorous, 2.2% chromium, 0.1% molybdenum, 0.1%nickel, 0.23% vanadium, 0.06% niobium, 0.05% titanium, 1.9% tungsten,0.009% nitrogen, 0.003% boron and 0.02% aluminium.

[0025] The rotor element may be formed from steel which comprises from0.15 to 0.35% carbon, from 0 to 0.3% silicon, from 0.2 to 1% manganese,from 0 to 0.03% sulphur, from 0 to 0.03% phosphorous, from 0.3 to 1%nickel, from 0.7 to 1.50% chromium, from 0.5 to 1.2% molybdenum, andfrom 0.2 to 0.4% vanadium.

[0026] In one particularly preferred embodiment, the rotor element isformed from steel comprising substantially 0.25% carbon, 0.23% silicon,0.64% manganese, 0.005% sulphur, 0.01% phosphorous, 0.56% nickel, 0.8%chromium, 0.78% molybdenum, and 0.35% vanadium.

[0027] Embodiments of a method and a rotor in accordance with theinvention, will now be described, by way of example and with referenceto FIG. 1, which shows creep rupture strengths for the described rotoraccording to the invention in comparison with examples of the prior art.

[0028] This embodiment is a repair process conducted on an early stagedisk head of a rotor.

[0029] The invention is not, of course, limited to use with thisparticular type of rotor component.

[0030] The rotor is formed from 1% CrMoV steel having a composition:

[0031] C: 0.25%

[0032] Si: 0.23%

[0033] Mn: 0.64%

[0034] S: 0.005%

[0035] P: 0.01%

[0036] Ni: 0.56%

[0037] Cr: 0.80%

[0038] Mo: 0.78%

[0039] V: 0.35%

[0040] Fe: 96.375%

[0041] The disk head has a creep-life expired region that is to berepaired and the metal in this region is therefore initially machinedaway to define a rotor element, upon which the welding process is to beperformed.

[0042] In this particular embodiment, the invention makes use of asubmerged-arc welding process, which is generally well known. Otherwelding processes could be employed, if preferred.

[0043] The weld metal, as is common in submerged-arc welding, takes theform of a consumable wire and, in this example, has the followingcomposition:

[0044] C: 0.075%

[0045] Si: 0.2%

[0046] Mn: 0.5%

[0047] S: 0.001%

[0048] P: 0.017%

[0049] Cr: 2.2%

[0050] Mo: 0.1%

[0051] Ni: 0.1%

[0052] V: 0.23%

[0053] Nb: 0.06%

[0054] Ti: 0.05%

[0055] W: 1.9%

[0056] N: 0.009%

[0057] Al: 0.02%

[0058] B: 0.003%

[0059] Fe: 94.535%.

[0060] Prior to the commencement of the welding, a layer of flux powderis placed on the surface of the rotor element. A bonded or fused fluxcould be used and the selection will depend upon the particular weldingcircumstances. This is not a feature of the invention and will,therefore, not be discussed further. As is usual, the welding iseffected by the formation of an arc between the wire electrode and therotor element. The arc is caused to move along the rotor element and, asit does so, excess flux is recycled via a hopper. Remaining fused slaglayers are removed after welding.

[0061] Following welding, a heat treatment at a temperature rangingbetween 650° C. and 750° C. is performed.

[0062] The final stage is to machine the weld metal.

[0063] The resulting weld has a high creep strength, which is similar tothat of the rotor steel. It also has physical properties which aresimilar to those of the rotor steel. Accordingly, the overall creepstrength of the resulting, repaired rotor is not compromised as aconsequence of the weld and the creep-life is highly predictable and canbe guaranteed for a relatively long period. Furthermore, there is nodisparity between the physical characteristics, and particularly theco-efficients of thermal expansion, of the rotor steel and the weldmetal, thereby avoiding detrimental stresses when thermal variationsoccur.

[0064]FIG. 1 shows the cross-weld strength of the welded metal in theabove embodiment, relative to Larson-Miller parameter (P). Forcomparison, FIG. 1 also shows the cross-weld strengths of the two knownweld metals discussed above (5% Cr and 2% CrMo) and the scatterband forthe 1% CrMoV steel from which the rotor element is made. It is clearfrom the figure that the strength for the embodiment of the inventionfalls well within the scatterband for the rotor, unlike the known weldmetals which offer lower strength, falling outside the scatterband.

[0065] It should be understood that the above embodiment is merely anexample of a process, in accordance with the invention, resulting in arotor in accordance with the invention.

[0066] In particular, it should be understood that the rotor steel mayhave any of a range of compositions and can comprise, inter alia:

[0067] C: from 0.15 to 0.35%

[0068] Si: from 0 to 0.3%

[0069] Mn: from 0.2 to 1%

[0070] S: from 0 to 0.03%

[0071] P: from 0 to 0.03%

[0072] Ni: from 0.3 to 1%

[0073] Cr: from 0.7 to 1.5%

[0074] Mo: from 0.5 to 1.2%

[0075] V: from 0.2 to 0.4%.

[0076] Similarly, the weld metal may have any of a range of compositionsand can comprise inter alia:

[0077] C: from 0.04 to 0.1%

[0078] Si: from 0 to 0.5%

[0079] Mn: from 0.1 to 0.6%

[0080] S: from 0 to 0.01%

[0081] P: from 0 to 0.03%

[0082] Cr: from 1.9 to 2.6%

[0083] Mo: from 0.05 to 0.3%

[0084] V: from 0.2 to 0.3%

[0085] Nb: from 0.02 to 0.08%

[0086] W: from 1.45 to 2.1%

[0087] N: from 0 to 0.03%

[0088] Al: from 0 to 0.03%

[0089] B: from 0.0005 to 0.006%.

[0090] Furthermore, within these ranges, a number of preferred rangesexist, as follows.

[0091] If the amount of carbon in the weld metal is 0.06% or more, ithas been found that strength characteristics of the weld areparticularly good.

[0092] If the amount of manganese in the weld metal is 0.3% or more orthe amount of sulphur is 0.005% or less, it has been found that thelikelihood of hot cracking is reduced. The effect is particularlypronounced if both conditions are met.

[0093] If the amount of tungsten in the weld metal is 1.7% or more, ithas been found that strength characteristics of the weld areparticularly good.

[0094] If the amount of niobium in the weld metal is 0.04% or more, ithas been found that strength characteristics of the weld areparticularly good.

[0095] If the amount of nitrogen in the weld metal is 0.02% or less, ithas been found that the level of porosity of the weld metal can be keptlow.

[0096] As will be apparent from the above example, nickel may also beincluded in the weld metal alloy, in which case, if it constitutes 0.5%or less, it has been found that the long term creep strength of the weldmetal can be maximised.

[0097] It should further be understood that the invention is not limitedfor use in the type of rotor discussed above. Its application can extendto most types of rotor, made from a wide range of steels.

[0098] Although the above repair was effected by machining away thecreep-expired material and replacing this material by weld metal; itcould alternatively be the case that the creep-expired material isreplaced by a further rotor element, which is welded to the originalelement. In such a case, the rotor weld elements could actually beintegrally formed, in some region other than the weld region, prior tothe weld. In other words, it is not necessary for the rotor elements tobe discrete components, they could be joined parts of a singlecomponent.

[0099] It is emphasised that the invention is not limited to the repairof rotors nearing the expiry of their creep-life, but is applicable toother types of repair and also manufacture. For example, the inventionis highly effective in the modification of rotors by weld build-up priorto service.

[0100] Many further modifications and variations will suggest themselvesto those versed in the art upon making reference to the foregoingillustrative embodiment, which is not intended to limit the scope of theinvention, that being determined by the appended claims.

1. A method of forming a rotor comprising the steps of: (i) providing arotor element formed from steel; and (ii) welding the rotor element,using a welding process employing a weld metal which comprises: from0.04 to 0.1% carbon, from 0 to 0.5% silicon, from 0.1 to 0.6% manganese,from 0 to 0.01% sulphur, from 0 to 0.03% phosphorous, from 1.9 to 2.6%chromium, from 0.05 to 0.3% molybdenum, from 0.2 to 0.3% vanadium, from0.02 to 0.08% niobium, from 1.45 to 2.1% tungsten, from 0 to 0.03%nitrogen, from 0.0005 to 0.006% boron and from 0 to 0.03% aluminium. 2.A method according to claim 1, wherein the said weld metal comprises atleast 0.06% carbon.
 3. A method according to claim 1 or 2, wherein thesaid weld metal comprises at least 0.3% manganese.
 4. A method accordingto any preceding claim, wherein the said weld metal comprises 0.005% orless sulphur.
 5. A method according to any preceding claim, wherein thesaid weld metal comprises at least 1.7% tungsten.
 6. A method accordingto any preceding claim, wherein the said weld metal comprises at least0.04% niobium.
 7. A method according to any preceding claim, wherein thesaid weld metal comprises 0.02% or less nitrogen.
 8. A method accordingto any preceding claim, wherein the said weld metal further comprises0.5% or less nickel.
 9. A method according to any preceding claim,wherein the said weld metal comprises substantially 0.075% carbon, 0.2%silicon, 0.5% manganese, 0.001% sulphur, 0.017% phosphorous, 2.2%chromium, 0.1% molybdenum, 0.1% nickel, 0.23% vanadium, 0.06% niobium,0.05% titanium, 1.9% tungsten, 0.009% nitrogen, 0.003% boron and 0.02%aluminium.
 10. A method according to any preceding claim, wherein therotor element is formed from steel which comprises from 0.15 to 0.35%carbon, from 0 to 0.3% silicon, from 0.2 to 1% manganese, from 0 to0.03% sulphur, from 0 to 0.03% phosphorous, from 0.3 to 1% nickel, from0.7 to 1.50% chromium, from 0.5 to 1.2% molybdenum, and from 0.2 to 0.4%vanadium.
 11. A method according to any preceding claim, wherein therotor element is formed from steel comprising substantially 0.25%carbon, 0.23% silicon, 0.64% manganese, 0.005% sulphur, 0.01%phosphorous, 0.56% nickel, 0.8% chromium, 0.78% molybdenum, and 0.35%vanadium.
 12. A method according to any preceding claim, comprisingproviding a second rotor element having a composition substantially thesame as the said rotor element and welding the said second rotor elementto the said rotor element using the said weld metal.
 13. A methodaccording to any preceding claim, wherein the said welding process is asubmerged-arc welding process.
 14. A method according to any precedingclaim, wherein the said method comprises a step of machining a rotorcomponent to form at least one of the said rotor elements.
 15. A methodaccording to any preceding claim, comprising a step of machining thesaid weld metal after the weld has been formed.
 16. A rotor for aturbine, comprising a rotor element and weld metal welded to the saidrotor element; wherein the weld metal comprises: from 0.04 to 0.1%carbon, from 0 to 0.5% silicon, from 0.1 to 0.6% manganese, from 0 to0.01% sulphur, from 0 to 0.03% phosphorous, from 1.9 to 2.6% chromium,from 0.05 to 0.3% molybdenum, from 0.2 to 0.3% vanadium, from 0.02 to0.08% niobium, from 1.45 to 2.1% tungsten, from 0 to 0.03% nitrogen,from 0.0005 to 0.006% boron and from 0 to 0.03% aluminium.
 17. A rotoraccording to claim 16, wherein the said weld metal comprises at least0.06% carbon.
 18. A rotor according to claim 16 or 17, wherein the saidweld metal comprises at least 0.3% manganese.
 19. A rotor according toany of claims 16 to 18, wherein the said weld metal comprises 0.005% orless sulphur.
 20. A rotor according to any of claims 16 to 19, whereinthe said weld metal comprises at least 1.7% tungsten.
 21. A rotoraccording to any of claims 16 to 20, wherein the said weld metalcomprises at least 0.04% niobium.
 22. A rotor according to any of claims16 to 21, wherein the said weld metal comprises 0.02% or less nitrogen.23. A rotor according to any of claims 16 to 22, wherein the said weldmetal further comprises 0.5% or less nickel.
 24. A rotor according toany of claims 16 to 23, wherein the said weld metal comprisessubstantially 0.075% carbon, 0.2% silicon, 0.5% manganese, 0.001%sulphur, 0.017% phosphorous, 2.2% chromium, 0.1% molybdenum, 0.1%nickel, 0.23% vanadium, 0.06% niobium, 0.05% titanium, 1.9% tungsten,0.009% nitrogen, 0.003% boron and 0.02% aluminium.
 25. A rotor accordingto any of claims 16 to 24, wherein the rotor element is formed fromsteel which comprises from 0.15 to 0.35% carbon, from 0 to 0.3% silicon,from 0.2 to 1% manganese, from 0 to 0.03% sulphur, from 0 to 0.03%phosphorous, from 0.3 to 1% nickel, from 0.7 to 1.50% chromium, from 0.5to 1.2% molybdenum, and from 0.2 to 0.4% vanadium.
 26. A rotor accordingto any of claims 16 to 25, wherein the rotor element is formed fromsteel comprising substantially 0.25% carbon, 0.23% silicon, 0.64%manganese, 0.005% sulphur, 0.01% phosphorous, 0.56% nickel, 0.8%chromium, 0.78% molybdenum, and 0.35% vanadium.